It is, but would most possibly fall into the category of English words with disputed usage.
The word foreshadowment has actually appeared in a number of literary works. Google Books did find books with content that contains a match for that word.
However, it appears that it has not been entered into the dictionary yet. Merriam-Webster has clearly defined the following related terms (but none for foreshadowment):
foreshadow - to give a suggestion of (something that has not yet happened)
foreshadowing - something believed to be a sign or warning of a future event
Erin McKean, in her article, Using Undictionaried Words notes:
Most of the words you know and love and use every day are not words you learned by looking them up in a dictionary and reading a definition. They’re words you learned by seeing them used by other writers. And that’s how dictionary editors work, too.
Scholars recently analyzed more than five million digitized books, about 4 percent of all the books ever printed. Publishing their findings in “Science,” the researchers discovered that, by their estimation, “52 percent of the English lexicon – the majority of the words used in English books – consists of lexical ‘dark matter’ undocumented in standard references.”